The present invention relates to an improved heat exchange structure, and a cooling device comprising such a structure, for example a heat pipe whereof said structure forms the evaporator.
The miniaturization of electronic components, and in particular of power components, poses the problem of integration of the devices for dissipating the heat given off by said components. In the microelectronics and power electronics field, the thermal fluxes to be dissipated are increasingly high and require an ever greater reduction of the thermal resistance. The thermal fluxes to be discharged often have average values around 150-200 W/cm2, but can reach 1000 W/cm2. For these heat flux density levels, the so-called passive cooling methods, such as vaned dissipaters or monophase liquid systems, quickly reach their power limit for power to be extracted.
Other solutions have therefore been imagined such as liquid/vapor diphase heat transfers. The technologies implementing these transfers are the heat pipe, thermosiphon, capillary loops or diphasic pumped loops. The component to be cooled is situated on the cold area of the cooling device, i.e. the evaporator.
One solution for cooling an electronic component is to implement a heat pipe that absorbs the heat at the component and discharges it towards the outside, this solution in particular being described in the document “Conception and test of flat heat pipe for 3D packaging cooling” Lora Kamenova, Yvan Avenas, Nathaliya Popova, Christian Schaeffer, Slavka Tzanova, 1-4244-0755-Sep. 7, 2007 IEEE, p. 787-792.
Heat pipes make it possible to increase the thermal conductivity of a heat dissipater. Heat pipes make it possible to transfer thermal power extracted from a given surface towards a secondary surface, often more accessible or offering better heat exchange for dissipation. The heat pipe is a closed system in which a liquid fluid is placed balanced with its vapor. The heat pipe comprises an area forming an evaporator on the side of the electronic component, where the heat to be dissipated is absorbed through vapor formation. The vapor produced to be evaporated migrates through the heart of the heat pipe to a condensation area where the absorbed heat is released by liquefaction of the vapor, the heat is therefore discharged. The condensate returns toward the evaporation areas owing to the capillary forces.
The heat pipe can be attached on the electronic component or integrated on the component, in the case of microelectronic chips, this consists of implanting an array of channels directly on the silicon substrate, thereby reducing the thermal resistance.
Another solution is to make a thermosiphon, the fluid being displaced by gravity, the evaporator being located below the condenser.
Still another solution is the diphasic pumping system using a motor system, a condenser and an evaporator.
All of these solutions therefore use a thermal exchange surface in contact with the component, either attached on the component to be cooled, or formed in the substrate of the component to be cooled, said surface being intended to extract the heat generated in the component. This extraction, as explained above, is done through a phase change by evaporation of a liquid.
The major problem in terms of this exchange surface is the risk of drying of the surface, which leads to a significant decrease of the heat exchange coefficient, and therefore a decrease in the amount of heat evacuated. The temperature of the heat exchange wall then increases, as well as that of the component, this temperature can then become critical for the component.
The drying corresponds to an absence of liquid at the heat exchange surface, which can occur due to excessively high heat fluxes creating a vapor bubble blocked in the channel in the case of a heat pipe, the surface on which the bubble is blocked no longer sees liquid, and the heat extraction cannot take place. This can also occur through rupture of the liquid film intended to be distributed homogeneously over the inner surface of the channel.
Moreover, it has been shown that the wetting surfaces have very good boiling heat transfer performances. For example, in the document Y. Takata, S. Hidaka, J. M. Cao, T. Nakamura, H. Yamamoto, M. Masuda, T. Ito, “Effect of surface wettability on boiling and evaporation”, Energy 30 (2005) 209-220, it is shown that the heat exchange coefficient of a hydrophilic surface obtained by TiO2 deposition is greatly increased, and its heat flow becomes critical relative to an untreated surface.
In the article S. Ujereh, T. Fisher, I. Mudawar, “Effects of carbon nanotube arrays on nucleate pool boiling”, Int. J. of Heat and Mass Transfer 50 (2007) 4023-4038, the heat exchange properties of more or less covered surfaces of carbon nanotubes have been studied, it has been observed that a surface completely covered with nanotubes offers better thermal performance relative to the partially covered surfaces.
Document S. Kim, H. Kim, H. D. Kim, H. S. Ahn, M. H. Kim, J. Kim, G. C. Park, “Experimental investigation of critical heat flux enhancement by micro/nanoscale surface modification in pool boiling”, ICNMM2008, Jun. 23-25, 2008, Darmstadt, Germany shows that a surface combining microstructures and nanostructures makes it possible to reduce the contact angle from 83° to 0° and to increase the critical heat flux by 200%.
Super wetting surfaces therefore appear to be particularly effective to obtain good thermal performance. However, the more the wettability of a surface is increased, the more the energy required to form the first nuclei of vapor is higher. As a result, the vapor bubble formation frequency is reduced in the case of wetting surfaces.
As a result, it is one aim of the present invention to offer a heat exchange device with improved performance, or more generally a heat exchange structure having increased effectiveness.